The invention relates to a LiaNixCoyMny′M′zO2 composite oxide (M′ being Al, Mg, Ti, Cr, V, Fe, Ga) with a non-homogenous Ni/M ratio in particles of different sizes, allowing excellent power and safety properties when used as positive electrode material in Li battery.
Due to their high energy density, rechargeable lithium and lithium-ion batteries can be used in a variety of portable electronics applications, such as cellular phones, laptop computers, digital cameras and video cameras. Commercially available lithium-ion batteries typically consist of graphite-based anode and LiCoO2-based cathode materials. However, LiCoO2-based cathode materials are expensive and typically have a relatively low capacity of approximately 150 mAh/g.
Alternatives to LiCoO2-based cathode materials include LNMCO type cathode materials. LNMCO means lithium-nickel-manganese-cobalt-oxide. The composition is LiMO2 or Li1+xM1−xO2 where M=NixCoyMny′M′z. LNMCO has a similar layered crystal structure as LiCoO2 (space group r-3m). The advantage of LNMCO cathodes is the much lower raw material price of the composition M versus Co. The preparation of LNMCO is in most cases more complex than LiCoO2, because special precursors are needed wherein the transition metal cations are well mixed. Typical precursors are mixed transition metal hydroxides, oxyhydroxides or carbonates. Typical LiNMCO-based cathode materials include compositions having a formula LiNi0.5Mn0.3Co0.2O2, LiNi0.6Mn0.2Co0.2O2 or Li1.05M0.95O2, with M=Ni1/3Mn1/3Co1/3O2. Compared with LiCoO2, LNMCO tends to have a lower bulk diffusion rate of lithium, which can limit the maximum possible particle size for a given composition. Depending on the composition, the safety of the charged cathode in a real cell can be a problem. Safety events ultimately are caused by reactions between the oxidized surface and the reducing electrolyte. Thus safety problems are more severe if the particles have a high surface area, which is the case if the particle size is small. The conclusion is that the lower performance of LNMCO requires a small particle size which deteriorates safety.
A way to improve the safety has been to dope LNMCO materials with inert elements such as Al, Mg, Ti, in order to stabilize the structure when heated in the charged state. A drawback to that major improvement regarding safety is the fact that inert element doping is detrimental for power and reversible capacity within the LNMCO material. In order for this material to be industrially usable, manufacturers had to find a compromise between safety and performance, thus using the lowest amounts of Al, Ti and Mg required for obtaining a satisfying safety, while keeping decent power and capacity performances. Recently there have been numerous disclosures about the influence of Mg and Al doping for LNMCO with Ni:Co:Mn=33:33:33, or other compositions as for example LiNi1−x−yMnxCoyO2. It is widely expected that such composition will become a commercial product soon. However, as explained above, these products typically suffer from a difficult compromise between safety and electrochemical performances, thus resulting in medium level of overall performances.
With the appearance of new applications for large batteries on the market (e.g. for hybrid vehicles or stationary power devices) and a need for meeting high safety requirements, without compromising on power performances, it appears that a breakthrough is needed in the synthesis of these NiMnCo-based materials.
As there has always been a concern to manufacture materials that are as homogeneous as possible, the state of the art manufacturing process of LiaNixCoyMny′M′zO2 (M′=Al, Ti, Mg . . . ) products uses doped precursors such as hydroxides (see for example in U.S. Pat. No. 6,958,139), carbonates, nitrates or oxides, that are sintered at temperatures above 600° C. Thus, the material is perfectly homogeneous in composition, and the resulting positive electrode material shows medium level of global performances. Considering fundamentals from solid state chemistry applied to battery materials, it is known that for LiCoO2 material, smaller particle size gives better power performances (as discussed in Choi et al., J. Power Sources, 158 (2006) 1419). It is however also known that a smaller particle size gives lower safety, as safety characteristics are somewhat linked to surface area (see for example Jiang et al., Electrochem. Acta, 49 (2004) 2661). It follows that for the LiNixCoyMny′M′zO2 system, where the presence of given amounts of Ni and M′ (M′ being e.g. Al) are focused respectively on improving power behaviour and safety, a homogenous composition both for small and large particles leads to a compromise between power and safety performance. Real powders have a distribution of particles with different size. However, a homogeneous composition of all particles is not preferred at all. For the small particles in which safety behaviour is directly related to M′ content, a higher M′ concentration would be needed to achieve the same safety behaviour as for larger particles. On the other hand, less M′ (inert doping) is needed in the large particles but a decrease of M′ in the large particles would enhance the performances of the LiNixCoyMny′M′zO2 system.
The present invention provides a solution to this problem.